1. Field of the Invention
The present invention concerns a new staple that can be easily used both as a staple and as an osteosynthesis plate specifically designed for tibia and femur osteotomies.
2. Discussion of Background Information
FR 2 642 641 describes a surgical staple in the form of a &lt;&lt;U &lt;&lt;, which has two parallel side legs having sharpened free ends and being joined by a cross member. The cross member is pierced by at least one hole approximately parallel to the side legs and intended to take at least one additional fixing device. The side legs of the staple have, firstly, at their end, a sharpened part pointing approximately towards the inside and, secondly, on their external surface a series of teeth, the ridges of which point approximately upwards. The internal part of the cross member between the side legs of the staple has a smooth, flat, horizontal surface. In addition, the cross member is connected to the side legs by internal angular sections. For one of its applications, the staple is used as an osteosynthesis device and, consequently, it is intended to be sunk into the bone in such a way that its cross member bears on the bone in the region of the fracture.
This staple clearly has a number of disadvantages for osteosynthesis, notably that, once it is in place, the staple tends to spread by reaction and consequently produce an opposite effect to that desired, which is to draw together and anchor the bone fragments to be fused. In fact, the sharpened section of each of the side legs of the staple has a point directed towards the interior which has the effect of forcing the legs towards the interior when penetrating the bone and thus causing an outwards reaction by elastic restitution. This reaction will in practice tend to separate the bone fragments. It should be noted that the arrangement of the series of teeth on the external surfaces of the side legs reinforces this separating effect by reaction. Also, if the staple does not have any additional fixing devices, it will be displaced not only by the progressive formation of callus (bony substance produced in the outer part of the bone which progressively fuses the bone fragments together) around the side legs, but also by the effect of the aforesaid reaction which naturally tends to eject it from the bone.
There is another bone surgery staple as described in Swiss patent CH-A-597 839. In this version the sharpened ends of the legs are such that the sloping surfaces forming the bevels face each other, which is a considerable advantage over the previous staple. However, once implanted, this staple does not provide any elastic compression of the bone fragments.
Moreover, none of the earlier designs takes account of the bone structure into which the staples are to be implanted. In particular, there is no mention in the previous documents of the fundamental differentiation between the very hard and brittle cortical bone and the somewhat soft and spongy metaphysis. Staples frequently have to be implanted lengthways in the bone, that is one leg in the metaphysis and the other in the cortical bone. This is notably the case for knee articulation surgery.
Another problem is that the cross member of such staples has a smooth, perfectly flat and horizontal internal surface. This geometry does not allow the internal surface of the cross member of the staples to mate properly with the generally concave surface of long bones which, apart from anything else, could have regions of discontinuity resulting notably from an osteotomy with derotation of the bone fragments.
Even when perfectly inserted into the bone, this type of staple always has an area which is not in contact with the bone. This failure to make contact is particularly serious for osteosynthesis: initially, it causes mechanical weakness and, additionally, the callus that will develop in the interstices will cause pressure on the staple which will be progressively displaced. This inevitably leads to a new operation with the undesirable consequences that can be imagined.
Finally there is another anterior patent U.S. Pat. No. 4.848,328 which descries an osteosynthesis staple whose side legs may be slightly divergent and whose ends are also provided with bevels, the sloping surfaces of which face each other for good reasons. This staple, for which a stepped variant is also described, is difficult to fit the shape of long bones and risks, in the same way as the previous solutions, being easily displaced. Finally, it is clear that this model of staple does not take account of the different natures of the bone into which the legs penetrate depending on whether the operation takes place in the metaphysis or the cortical bone.